Author’s note: Security Council Report (SCR) is an independent think tank dedicated to supporting a more effective, transparent, and accountable U.N. Security Council. A version of this article will appear in SCR’s October Monthly Forecast.
In recent years the use of force in self-defense has become increasingly contentious. Article 51 of the United Nations Charter, which recognizes member States’ right of self-defense against an armed attack, sits at the heart of these debates. It provides that:
Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken the measures necessary to maintain international peace and security.
Action taken by member States in exercise of this right is one of only two exceptions to the general prohibition on the use of force enshrined in Article 2(4), which the International Court of Justice (ICJ) has described as “the cornerstone of the United Nations Charter” (see Democratic Republic of the Congo v. Uganda, para 148). The application of Article 51 is therefore critically important to the collective security system established by the Charter. (The other exception that must be noted, of course, is the use of force authorized by the U.N. Security Council pursuant to Chapter VII of the Charter)
There has been significant disagreement among member States, academics and other commentators, however, regarding the interpretation of Article 51 and its precise scope. These debates have intensified over the last decade, particularly as the United States and its allies have increasingly relied on the expansive “unwilling or unable” doctrine to justify military measures on the territory of other States, including counter-terrorism operations.
While these debates are not new, they have assumed renewed significance in light of recent developments. In the last few months alone, there have been several high-profile examples of member States using force in purported exercise of their right of self-defense, either explicitly or implicitly. These examples push the boundaries of this right to their limits, even for those who take a more expansive view of where they lie. They are also part of a broader trend. In February 2025, Mexico noted that since 2021, Article 51 has been invoked on at least 78 occasions, a marked increase compared to previous years.
Israel’s Sept. 9 strike against Hamas leadership in Qatar is one such example. In a joint statement issued on the same day as the strike, Israeli Prime Minister Benjamin Netanyahu and Defense Minister Israel Katz used language suggesting that Israel was acting in self-defense, saying that the action was justified due to Hamas’s actions on October 7th and its ongoing attacks on Israel.
The U.S. strikes against Iranian nuclear facilities on June 22 are another example. In an Article 51 report sent to the president of the Security Council following the strikes, the United States noted that it had taken “necessary and proportionate” action to defend Israel and its own security in accordance with Article 51. It also said that “peaceful measures were exhausted” and accused Iran of refusing to negotiate in good faith.
Israel’s related attack on Iran on June 13 is yet another example. While a June 17 letter from Israeli Foreign Minister Gideon Sa’ar to the president of the Security Council regarding the attack did not directly cite Article 51, it used language suggesting that Israel was acting in self-defense. The letter noted that the attacks were launched “in response to, and in order to thwart, the threat of imminent Iranian attacks” and said that Israel acted “to defend its security and very existence.”
The Sept. 2 U.S. strike against a vessel purportedly involved in drug trafficking in the Caribbean Sea is a fourth example. In his war powers resolution report to Congress, U.S. President Donald Trump directly invoked the right of self-defense and the unwilling or unable doctrine, saying:
In the face of the inability or unwillingness of some states in the region to address the continuing threat to US persons and interests emanating from their territories, we have now reached a critical point where we must meet this threat to our citizens and our most vital national interests with United States military force in self-defense.
Several leading academics in the field, including a former State Department attorney and a professor of international law who has argued in favor of the unwilling or unable doctrine, have raised concerns regarding the invocation of the right of self-defense in these examples.
The Security Council has an important role to play in facilitating consideration of these questions. In addition to recognizing member States’ right of self-defense, Article 51 also imposes the following reporting requirement:
Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take such action as it deems necessary in order to maintain or restore international peace and security.
The concerns raised by experts highlight the significance of Article 51 reports and the importance of the Security Council’s role. Given the rise in interstate conflict in recent years, and the more frequent references to Article 51 as a rationale for the use of force, ready access to information regarding action taken under Article 51 is crucial in order for the Security Council and other member States to evaluate and respond to claims that States are acting in self-defense. Although the reporting requirement appears clear enough on its face, implementation has been largely inconsistent. This has negative consequences for both the Security Council’s work and the development of international law.
This article examines some of the differing positions of member States in relation to the scope of the right of self-defense, analyzes relevant practice regarding the reporting requirement set out in Article 51 and the implications of that practice, and offers suggestions for improvements in this area.
Differing Member State Positions on Article 51
Member States disagree over several aspects of Article 51. To begin with, a significant number of States take the view that an armed attack must occur before a member State can exercise its right of self-defense. These States tend to argue that the wording of Article 51 is clear and should not be reinterpreted, and often claim that ICJ jurisprudence, including the decisions in Nicaragua v USA, DRC v Uganda, Oil Platforms, and the Israeli Wall Advisory Opinion, supports their view. This position was recently reaffirmed in the outcome document of the 19th high-level summit of the Non-Aligned Movement (NAM) held in January 2024.
A smaller group of States, which largely comprises the United States and its allies, have adopted a more expansive position, arguing that the right of self-defense can be exercised in certain circumstances where an armed attack is imminent but has not yet occurred. A 2016 report issued by the White House is often cited by commentators as evidence of this position.
The “unwilling or unable” doctrine, which posits that member States may use force against non-State actors on the territory of another State if that State is unwilling or unable to prevent attacks originating from its territory, has also sparked significant disagreement among States. Article 51 reports sent to the Security Council by the United States and several of its allies invoking the doctrine as the basis for military operations on the territory of other States have sometimes proven particularly controversial, leading to pushback from other member States.
These divisions were on display during an Arria-formula meeting on Article 51 convened by Mexico in February 2021. A number of participants expressed support for a broad conception of the unwilling or unable doctrine, including Australia, which said that the right of self-defense can be exercised against non-State actors in the territory of another State if that State is unwilling or unable to prevent attacks originating from its territory, without offering additional qualifications.
Other participants articulated a narrower formulation of the doctrine. Austria, for example, said that Article 51 can be invoked to justify the use of force against a non-State actor on the territory of another State if two conditions are satisfied: 1) a “transboundary element” and 2) the host State is “harbouring or otherwise substantially supporting” the non-State actor or is unable to prevent its operations because of a lack of State authority and effective control in respect of the relevant territory.
Although many participants expressed support for using the doctrine as a basis for invoking Article 51, others directly rejected this idea. Mexico argued that it is not legally sound and noted that the doctrine is not supported by the text of Article 51, while also suggesting that it could afford a unilateral margin of interpretation to States that rely on it and lead to unforeseen consequences. China was similarly forthright in its statement, saying that force can only be used against non-State actors on the territory of another State with that State’s consent and criticizing the “use of force arbitrarily in the name of ‘preventive self-defence.’”
Reporting to the Security Council under Article 51
Given these disagreements and the recent invocations of the right of self-defense described above, the reports to the Security Council required by Article 51 are crucial. As a starting point, they provide a basis for council members to evaluate often contentious claims that a member State is acting in self-defense and determine whether and how the council should respond. This is especially important because Article 51 provides that member States may only exercise their right of self-defense until the council has taken the necessary measures to maintain international peace and security.
Article 51 reports also have a significant role to play, at least in theory, in the development of international law and interpretation of the U.N. Charter. As a treaty, the Charter is subject to the rules of treaty interpretation set out in the Vienna Convention on the Law of Treaties (VCLT). Pursuant to Article 31(3)(b) of the VCLT, subsequent State practice which establishes the agreement of the parties to the Charter must be taken into account when interpreting its provisions. This means that State practice regarding Article 51, including what States say when other States invoke it, has a direct bearing on the meaning of Article 51 and its scope as a matter of international law. The potential legal significance of Article 51 reports was recognized by the ICJ in its Nicaragua v USA judgment, albeit in the context of customary international law. The judgment noted that “the absence of a report may be one of the factors indicating whether the State in question was itself convinced that it was acting in self-defense”.
The text of Article 51 does not stipulate the form of Article 51 reports or the information that they should contain. In the early days of the Security Council, member States would sometimes request a meeting so they could provide an oral report regarding action taken in self-defense. Since then, however, the common practice has been for member States to report under Article 51 through a letter to the Security Council president that later becomes a public document, often within days of the letter being sent.
Such reports have sometimes provided a fulsome account of the actions taken by a member State in exercising its right of self-defense and its reasons for doing so, including the legal justification. When detailed information is reported, Article 51 reports provide a means for Security Council members and the wider U.N. membership to evaluate and respond to claims that a member State is acting in self-defense. Together with the Article 51 report itself, these responses can be analyzed and documented by academics, whose work can serve as a subsidiary means for assessing the scope of Article 51 in accordance with Article 38 of the ICJ statute.
Despite the significance of Article 51 reports to both the council’s work and international law, the relevant practice has been notably inconsistent. Member States do not always report action taken in the purported exercise of their right to self-defense, and when they do, the content of the reports can vary widely.
Several examples help to illustrate this point. Between 2014 and 2016, a number of States participated in military operations targeting ISIL on Syrian territory following a request from Iraq. While several Article 51 reports were sent to the council in respect of these operations, there were notable differences in the information that they contained. The United States, which sent the first report on Sept. 23, 2014, set out the details of Iraq’s request, described the threat posed by ISIL to Iraq and other States, and clearly indicated that it was acting on the basis of the “unable or unwilling” doctrine. The US report also claimed that the actions it had taken were necessary and proportionate and described its operations in general terms. The U.K. and France, conversely, sent reports that referred to the Iraqi request and noted that they had taken action in Syria with little additional detail or justification, while Canada provided a more detailed report that also referred to the “unable or unwilling” doctrine.
More recently, Russia sent a letter to the council regarding its invasion of Ukraine in February 2022 that referred to Article 51 and annexed a speech delivered by Russian President Vladimir Putin in Moscow on the day of the invasion. In July this year, Cambodia and Thailand each sent letters to the council regarding violent clashes along the disputed part of the border between the two countries. While both letters were relatively detailed regarding the military action taken and the justification for it, Thailand specifically referred to Article 51 whereas Cambodia did not. It instead stated that it was acting in self-defense without mentioning Article 51. India, for its part, did not send a formal communication to the Council regarding Operation Sindoor, which was carried out in Pakistani territory in May in response to the 22 April terrorist attack in Jammu and Kashmir, despite using language suggesting that it was acting in self-defense in official statements.
In addition, there is little consistency in the way the council responds to Article 51 reports. They do not automatically trigger a council meeting, and while the issues raised in the reports are usually discussed informally by council members’ legal advisers, there is no guarantee that the discussion will progress beyond that stage. If the wider issue that the report is related to is sufficiently high profile then aspects of the report may be discussed during a council meeting, but when this happens consideration of Article 51 is often subsumed by a broader political discussion, and the report may not be addressed directly.
Member States have complained that these inconsistencies have made it difficult to identify and access Article 51 reports, with some calling for a clearer, timely system for distributing them to States outside the Security Council. Mexico, which has been particularly active in relation to this issue, has gone further and suggested that the lack of clarity surrounding Article 51 reports and the council’s response to them has inhibited member States’ ability to adequately react when other States claim to be acting in self-defense.
Options for Possible Improvements
Given the importance of these reports, the relevant practice arguably leaves much to be desired. How, then, could it be improved? One possibility would be for Security Council members with an interest in this issue to hold another Arria-formula meeting with a specific focus on the interpretation of Article 51. This could be particularly useful given the uptick in reporting under Article 51 since the last Arria meeting on this issue was held in February 2021. Member States could also be encouraged to provide a public briefing to the council following a letter on the use of Article 51.
On the issue of Article 51 reports, the Informal Working Group on Documentation and Other Procedural Questions (IWG) could be tasked with developing guidelines for what should be included in such reports, when the reports should be sent to the council, and how the council should respond when they are received. If agreement is reached, those guidelines could be codified in a presidential note, a consensus document issued by the council president. The Security Council Affairs Division could also take the lead on developing a publicly accessible database which documents past reports and is updated in real time as new reports are received.
Members of the Accountability, Coherence and Transparency Group (ACT), a cross-regional group of 27 small and mid-sized States that aims to improve the work of the council, may wish to consider focusing on this issue as well, particularly given recent developments.
An advisory opinion from the ICJ regarding the scope of Article 51 could also be requested either by the Security Council or the General Assembly.
It should always be borne in mind that Article 51 provides an exception to the general prohibition on the use of force enshrined in Article 2(4), a peremptory norm of international law from which no derogation is permitted. States seeking to avail themselves of an exception to a peremptory norm should explain what they have done and why and provide as much detail as sensitive military operations allow, particularly when it comes to the legal justification for those operations. Failure to improve the practice surrounding Article 51 reports carries significant risk in the current geopolitical climate.
